linux-kernel - [RFC] Common clock framework for external clock generators

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Date:	Thu, 10 May 2012 03:11:55 +0200
From:	Sebastian Hesselbarh <sebastian.hesselbarth@...glemail.com>
To:	linux-kernel@...r.kernel.org
Subject: [RFC] Common clock framework for external clock generators

Hi,

first of all I apologize for the quite long attachment but I think
it is useful for the following discussion.

I recently read about the newly introduced common clock framework (ccf)
and wondered if this could be also used for external, e.g. i2c attached,
clock generators.

Based on my current understanding of the framework I wrote such a
driver and now I want to present it here for clarification of some
remarks I have regarding the framework itself.

Please do not see this driver as mature but as some kind of
proof-of-concept. I have the driver somewhat running but stumbled
upon some issues.

First I want to give a brief overview of the intended use case of
this driver:

It is a driver for a clock generator that is externally attached to
a Marvell Dove (arm/mach-dove) SoC. It will provide driver configurable
clocks that are connected to dedicated clock inputs of the SoC, e.g.
external audio clock for i2s controller.

The basic intention I had in mind when writing this driver was to
add it during platform init and pass a list of clock aliases and clock
hierarchy description to allow the receiving driver, e.g. i2s, to set
the rate of the supplied clock without poking the clock generator
directly.

Please comment on the following aspects:
- there is no clk_unregister which is okay for platform clocks but
should be there since clock generators can be detached
- the clock generator has two plls and up to 8 clocks; inside the
clk_ops it is quite hard to find the correct struct clk_hw when
using container_of()
- most of clk_ops are spin-locked but i2c drivers tend to sleep
during read or write; this causes a "BUG: scheduling while atomic"

I know that ccf is quite new but it is well suited for generic,
i.e. platform independent, external clock generator drivers. Maybe
I got the overall concept just wrong but maybe this RFC helps
to straighten things up for future drivers.

Regards,
     Sebastian

View attachment "clk-si5351.lkml.c" of type "text/x-csrc" (10706 bytes)